1. Field of the Invention
The present invention relates to digital-to-analog converters; and more specifically, high output power digital-to-analog converters.
2. Description of Related Art
Conventional digital-to-analog converters DACs, such as for an ADSL modem, cannot drive the desired load. As a result, designers of DACs resort to using a linear driver stage (e.g., an amplifier) to boost the operating current and voltage. However, linear amplification of current and voltage with AC components that extend out to 2 MHz is difficult and inefficient. Designers of the linear amplifiers make tradeoffs that harm distortion, drive level, or frequency response because of the device limitations and cost constraints. It is possible to achieve low distortion, but the cost in power consumption is very high. Efficiency on the order of 10% is common with these linear techniques.
The digital-to-analog converter of the present invention eliminates the need for a linear amplification stage, and provides a simple and efficient conversion out to the 2 MHz range. Unlike conventional DACs that require converting the digital data to a current and then converting the current to an output voltage or require performing a current steering operation, the DAC of the present invention directly converts the digital data to an output voltage.
In a preferred embodiment, control signals representing the digital data are supplied to respective converting elements. Each converting element or converter selectively applies a voltage to an output line in response to the respectively received control signal. More specifically, the converter includes a switch that selectively connects the output line via a resistor to a voltage line having a predetermined voltage.
Furthermore, in the preferred embodiment, signal dependent error between adjacent samples, which shows up as harmonic distortion in the analog output signal, is eliminated. The preferred embodiment of the DAC according to the present invention includes reset circuitry that applies a zero value digital signal, preferably at the switches that selectively connect the output line via a resistor to a voltage line having a predetermined voltage. This zero value signal has the effect of adjusting the analog output signal to a reset voltage level between samples. This eliminates the above-mentioned harmonic distortion.
The system for performing digital-to-analog conversion, which includes the DAC of the present invention, also allows reduction in the number of bits representing a sample. In the system according to the present invention, the noise in the digital data is shifted to the upper frequency range, away from the desired frequency spectrum, and then filtered out from the analog output. Because of this noise shaping and filtering, fewer bits are required to obtain the same signal-to-noise ratio as compared to a lack of noise shaping and filtering.